Package Naming

For modules the package name should begin with perl- and the rest of the name should be constructed from the module name by converting it to lowercase and then replacing colons with hyphens. For example the package name corresponding to HTML::Parser will be perl-html-parser. Perl applications should have the same name as that of the application but in lowercase.

File Placement

Perl modules should install module files in /usr/lib/perl5/vendor_perl/ (this is done by setting the INSTALLDIRS variable as shown below). No files should be stored in /usr/lib/perl5/site_perl/ as that directory is reserved for use by the system administrator to install Perl packages outside the package management system. The files perllocal.pod and .packlist also should not be present; this is taken care of by the example PKGBUILD described below.

Notes

In most cases, you should put any in the arch array since most Perl packages are architecture independent.

Example

An example PKGBUILD can be found at /usr/share/pacman/PKGBUILD-perl.proto, which is in the abs package.

Automation

As Perl is centered around the CPAN, there are a few scripts to make the most of this, and save you writing PKGBUILDs by hand.

A plugin for the second-generation CPAN shell, CPANPLUS, is available in the perl-cpanplus-dist-arch package from the community repo. This plugin packages distributions on the fly as they are installed by CPANPLUS. Online documentation is available at https://metacpan.org/release/CPANPLUS-Dist-Arch

Warning: Pacpan development has been officially discontinued: its latest version does not work with pacman>=3.5. See [1].

It is worth mentioning that Bauerbill has similar support for generating PKGBUILDs to pacpan. As well as adding the ability to upgrade all installed CPAN modules directly from CPAN via a pacman interface. Make sure to read the Bauerbill man file for usage instructions.

Warning: Bauerbill development has been officially discontinued: its latest version does not work with pacman>=3.5. See [2].

Module Dependencies

Perl has a unique way of defining dependencies compared to similar systems like python eggs and ruby gems. Eggs define dependencies on other eggs. Gems depend on gems. Perl dists depend on modules. Modules are only available from CPAN distributions so in a way perl distributions depend on distributions only indirectly. Modules can define their own versions independent from distributions inside the module source code. This is done by defining a package variable called $VERSION. When using strict and warnings, this is defined with the our keyword. For example:

package Foo::Module;
use warnings;
use strict;
our $VERSION = '1.00';

Modules can change their versions however they like and even have a version distinct from the distribution version. The utility of this is questionable but it is important to keep in mind. Module versions are more difficult to determine from outside of the perl interpreter and require parsing the perl code itself and maybe even loading the module into perl. The advantage is that from inside the perl interpreter module versions are easy to determine. For example:

use Foo::Module;
print $Foo::Module::VERSION, "\n";

Implementation

The CPAN is a Centralized Network for Perl Authors. Each CPAN mirror contains indices that list the distributions on CPAN, the modules in the dists, and the name of the author who uploaded the dist. These are simply text files. The most useful index is in the /modules/02packages.details.txt.gz file available from each CPAN mirror. The term "packages" here refers to the package keyword in the perl language itself, not something similar to pacman packages. The CPAN shell, referred to as lowercased, italicized cpan, is simply the venerable perl script which navigates indices to find the module you want to install.

Modules are found in the 02packages.details.txt.gz list. On the same line as the module/package name is the path to the distribution tarball that contains the module. When you ask cpan to install a module, it will look up the module and install the relevant distribution. As the distribution is installing it will generate a list of module dependencies. Cpan will try to load each module dependency into the perl interpreter. If a module of the given version cannot be loaded the process is repeated.

The cpan shell does not have to worry about what version of the required module it is installing. cpan can rely on the fact that the latest version of the module must satisfy the requirements of the original module that it began installing in the first place. Only the latest versions of modules are listed in the packages details file. Unfortunately for the perl package author, we cannot always rely on the fact that our packages offer the most recent version of a perl distribution and the modules contained within. Pacman dependency checking is much more static and strongly enforced.

Dependency Definition

Where are dependencies defined in perl distributions? They are defined inside of the Makefile.PL or Build.PL script. For example, inside of the Makefile.PL script the WriteMakeFile function is called to generate the Makefile like this:

This is a contrived example but it is important to understand the dependencies aren't final until after the Makefile.PL or Build.PL script is run. Dependencies are specified at runtime, which means they can be changed or modified using the full power of perl. This means the module author can add, remove, or change versions of dependencies right before the distribution is installed. Some modules authors use this to do overly clever things like depend on modules only if they are installed. Some multi-platform dists also depend on system-specific modules when installed on different operating systems.

As an example, the CPANPLUS distribution looks for CPANPLUS::Dist plugins that are currently installed. If any plugins are installed for the currently installed version of CPANPLUS it adds them to the new CPANPLUS's prerequisites. I'm not quite sure why. Luckily for the perl packager most dependencies are static like in the above example that requires the POSIX module with a minimum version of 0.01.

Meta Information

Meta files are included in recent distributions which contain meta-information about distributions such as the name, author, abstract description, and module requirements. Previously there were META.yml files in the YAML format but more recently the switch has been made to META.json files in the JSON format. These files can
be edited by hand but more often they are generated automatically by Makefile.PL or Build.PL scripts when packaging a distribution for release. The latest specification is described in CPAN::Meta::Spec's online docs.

Remember that dependencies can be changed at runtime! For this reason another meta file is generated after running the build script. This second meta file is called MYMETA.json and reflects changes the script made at runtime and may be different from the meta file generated when the distribution was packaged for CPAN.

Elderly distributions on the CPAN have no meta file at all. These old releases predate the idea of the META.yml file and only describe their prerequisites in their Makefile.PL.

Perl and Pacman Versions

In perl versions are numbers. In pacman versions are strings.

Perl allows both decimal versions, like 5.002006, and dotted decimal versions, like 5.2.6. You might have a hard time comparing between decimals and dotted decimals, so perl converts dotted decimal versions to decimal versions by padding with zeros. Each dot separates up to three digits and 5.2.6 becomes 5.002006. Now its easy to compare with some simple arithmetic! The internal docs for the version.pm module describes the conversion of dotted decimal versions in more detail.

The important thing is that perl compares versions exactly the same as it compares two numbers. Versions are numbers. 5.10 = 5.1. Pretty easy right? Dotted decimals are not so easy. 5.1.1 == 5.001001. Huh? The bad part is most other systems think of versions as strings making perl the odd man out.

Pacman works best with dotted decimal versions and doesn't compare them numerically. Components are split at non-alphanumeric characters and compared side-by-side as integers. The first component that is not equal determines which version string is less than or greater the other. Right away the component with the longest length, as a string, is considered the largest. This means that 5.0001 is greater than 5.1. 5.10 is not equal to 5.1, it is greater.

The problem is that changing the length of the version string can seriously confuse pacman. Consider the releases of AnyEvent:

6.01 (2011-08-26)

6.02 (2011-08-26)

6.1 (2011-10-24)

6.11 (2011-11-22)

6.12 (2011-12-12)

That 6.1 in the middle can cause problems because the length of the version string has decreased. In pacman's world 6.1 is less than 6.02. If a package depends on perl-anyevent>=6.02 and only 6.1 is available in the repository, then pacman would be unable to fulfill the dependency.

One solution to this problem is to pad the pkgver with zeroes. Dependencies might also have to be padded with zeroes to make pacman happy. Version 6.1 would become 6.10.

Advanced Topics

If the packager has become comfortable enough with creating perl packages, the below sections may offer some new ideas to consider. This information might also help troubleshooting packaging problems.

Glossary

You should be familiar with the following terms.

Module

Modules are declared with the package keyword in perl. Modules are contained inside a .pm ("dot-pee-em") file. Though it's possible more than one module (package) is in the file. Modules have namespaces separated with :: (double colons), like: Archlinux::Module. When loading a module, the ::s are replaced with directory separators. For example: Archlinux/Module.pm will be loaded for the module Archlinux::Module.

Core Module

Core modules are included with an installation of perl. Some core modules are only available bundled with perl. Other modules can still be downloaded and installed separately from CPAN.

Distributions

(aka dist, package) This is the equivalent of an Archlinux package in CPAN-lingo. Distributions are .tar.gz archives full of files. These archives contain primarily .pm module files, tests for the included modules, documentation for the modules, and whatever else is deemed necessary.

Usually a distribution contains a primary module with the same name. Sometimes this is not true, like with the Template-Toolkit distribution. The latest package, Template-Toolkit-2.22.tar.gz, for the Template-Toolkit dist, contains no Template::Toolkit module!

Sometimes because distributions are named after a main module, their names are used interchangeably and they get muddled together. However it is sometimes useful to consider them a separate entity (like in Template-Toolkit's case).

User-Installed perl

A subtle problem is that advanced perl programmers may like to have multiple versions of perl installed. This is useful for testing backwards-compatibility in created programs. There are also speed benefits to compiling your own custom perl interpreter (i.e. without threads). Another reason for a custom perl is simply because the official perl ArchLinux package sometimes lags behind perl releases. The user may be trying out the latest perl... who knows?

If the user has the custom perl executable in their $PATH, the custom perl will be run when the user types the perl command on the shell. In fact the custom perl will run inside the PKGBUILD as well! This can lead to insidious problems that are difficult to understand.

The problem lies in compiled XS modules. These modules bridge perl and C. As such they must use perl's internal C API to accomplish this bridge. Perl's C API changes slightly with different versions of perl. If the user has a different version of perl than the system perl (/usr/bin/perl) then any XS module compiled with the user's perl will be incompatible with the system-wide perl. When trying to use the compiled XS module with the system perl, the module will fail to load with a link error.

A simple solution is to always use the absolute path of the system-wide perl interpreter (/usr/bin/perl) when running perl in the PKGBUILD.

Installation Modules

One of perl's greatest advantages is the sheer number of modules[/dists] available on CPAN. Not too surprisingly, there are also several different modules used for installing... well... modules! TMTOWTDI! I am not aware of a standard name for these types of modules, so I just called them "Installation Modules".

All these modules are concerned with is building the package and installing wherever the user wants. This seems straightforward, but considering the number of different systems perl runs on, this can get complex. These modules all place a perl code file inside the dist tarball. Running this perl script will initiate the build/installation process. I have called this the "Build script" in the below list.

ExtUtils::MakeMaker

The original, oldest module for installing modules is ExtUtils::MakeMaker. The major downside to this module is that it requires the make program to build and install everything. This may not seem like a big deal to linux users but is a real hassle for Windows people! In the name of progress the perl community is trying to encourage people to use the newer modules instead.

Module::Build

The main advantage of Module::Build is that it is pure-perl. This means it does not require a make program to be installed for you to build/install modules. Its adoption was rocky because if Module::Build was not already installed, you could not run the bundled Build.PL script! This is not a problem with recent versions of perl because Module::Build is a core module.

Module::Install

Another modern build/installation module, Module::Install still requires the make program be installed to function. It was designed as a drop-in replacement for ExtUtils::MakeMaker, to address some of MakeMaker's shortcomings.

One very interesting feature is that Module::Install bundles a completecopy of itself into the distribution file. Because of this, unlike MakeMaker or M::B, you do not need Module::Install to be installed on your system.

Another very unique feature is auto-install. This appears to be not recommended, but seems used quite often. When the module author enables auto-install for his distribution, Module::Install will search for and install any pre-requisite modules that are not installed when Makefile.PL is executed. This feature is skipped when Module::Install detects it is being run by CPAN or CPANPLUS. However, this feature is not skipped when run inside... oh I don't know... a PKGBUILD! I hope you can see how a rogue perl program downloading and installing modules willy-nilly inside a PKGBUILD can be a problem. See the #PERL_AUTOINSTALL environment variable to see how to fix this.

Environment Variables

A number of environment variables can affect the way the modules are built or installed. Some have a very dramatic effect and can cause problems if misunderstood. An advanced user could be using these environment variables. Some of these will break an unsuspecting PKGBUILD or cause unexpected behavior.

PERL_MM_USE_DEFAULT

When this variable is set to a true value, the installation module will pretend the default answer was given to any question it would normally ask. This does not always work, but all of the installation modules honour it. That doesn't mean the module author will!

PERL_AUTOINSTALL

You can pass additional command-line arguments to Module::Install's Makefile.PL with this variable. In order to turn off auto-install (highly recommended), assign --skipdeps to this.

export PERL_AUTOINSTALL='--skipdeps'

PERL_MM_OPT

You can pass additional command-line arguments to Makefile.PL and/or Build.PL with this variable. For example, you can install modules into your home-dir by using:

export PERL_MM_OPT=INSTALLBASE=~/perl5

PERL_MB_OPT

This is the same thing as PERL_MM_OPT except it is only for Module::Build. For example, you could install modules into your home-dir by using:

export PERL_MB_OPT=--install_base=~/perl5

MODULEBUILDRC

Module::Build allows you to override its command-line-arguments with an rcfile. This defaults to ~/.modulebuildrc. You can override which file it uses by setting the path to the rcfile in MODULEBUILDRC. The paranoid might set MODULEBUILDRC to /dev/null... just in case.

Hardened Example

Using all of our new accumulated knowledge, we can create a more hardened PKGBUILD that will resist any environment variables' attempts to sabotage it: